An attempt to reach a good aerodynamic performance of the transition part, is to choose the chord near the rotor axis longer than conventional as is described in PCT/EP2004/003294 and PCT/EP03/05605 (Wobben). According to the manufacturer, this option results in a yield increase in the order of 10-15%, however the embodiment is complex, heavy and therefore expensive. Furthermore the total surface of the rotor increases which leads to inter alia higher wind loads, in particular at the survival wind speed, which means further additional costs for the tower and foundation. A variant of this option is described in EP2194267A2 (GE), for which it is also the case that large, expensive and complex additions are required to capture the extra yield. Further developments can be found in US20120134836A1 (GE) and US20120141281A1 (GE), for which always applies that it considers large additions to the blade which leads to large costs and structural problems. An alternative transition part is described in U.S. Pat. No. 6,910,867B2 (Corten) and is also included in EP2343450A1 (LM). Here it is proposed to improve the aerodynamic performance of the part near the blade root to attach flow manipulators such as vortex generators and stagnation strips. This solution is simple and delivers 1½% additional yield, although the majority of the aerodynamic loss in the rotor center persists.
A variant of this solution is described US8052394B2 (Repower), wherein the strip is replaced by a large flow blocking element added to the blade which also characterizes this addition as complex, expensive and risky. Risky references to the probability that a part fixed to the blade root comes loose. Other embodiments of the transition part are described in US2011/0229332 A1 (Nordex) and US2012/0027588A1 (GE). In the first case an addition is proposed which deviates much from a structurally optimal shape. In the second case an addition or extension is proposed of e.g. 10% of the blade length which is fixed to the blade root. This also is a complex, expensive and risky solution. The application of flow blocking Gurney flaps to wind turbine profiles is studied manifold and proves to deliver a lift increase but also a loss due to higher drag. US20100141269A1 (GE) further describes such a flow blocking device. Herein a solution is presented for the fixation of Gurney flaps, which are proposed as flat back folded plates. The problem hereof is that such shapes have a high stiffness and therefore attract forces and thus easily come loose from the blade. Variants of the Gurney flap are additions such as e.g. are described in EP1845258A1 (Siemens) and US2009/0263252A1 (Gamesa). With those additions in the shape of diverging trailing edges, a lower drag is claimed, however compared to profiles without additions one does not reach considerably more lift, so that a disadvantageous large chord is required to achieve maximum efficiency and furthermore the flat trailing edges are structurally and production technically disadvantageous and therefore expensive.
Since the moment of the aerodynamic forces increases with decreasing distance to the shaft and because the area swept by a blade section and thus also the share in the energy yield decreases with decreasing distance to the shaft, a blade cross section is optimized from essentially aerodynamic to essentially structural in the direction from the tip to the axis. This means that profiles near the rotor axis are relatively thick with the disadvantage of aerodynamic loss. The wish of the structural engineer would be to choose starting from the blade root up to 80% radial position a profile thickness which is (much) more than the aerodynamically optimal thickness of 15-18% to create more building height so that a lighter blade design is achievable. However the ratio between lift and drag decreases with increasing relative profile thickness and furthermore the roughness sensitivity increases much with increasing relative profile thickness. Still the designer will be forced to apply thick airfoils for structural reasons with a reduction of yield as consequence.
A complication of the problem with wind turbine blades is that the optimum chord essentially is inversely proportional to the radial position and that a large chord is required at the blade root side of the blade to optimally extract energy from the flow. At the same time the swept area near the rotor center is relatively small so that the relative contribution to the yield of the inner part of the blade is small. The needed large chord and the relatively low contribution to the yield are reason for many manufacturers to choose the chord near the blade root shorter than optimal, so that the yield is reduced.
Another complication arrives from the twist of the angle of attack of the air flow to the profile in the direction from the tip to the axis: from more in the rotor plane to almost parallel to the axis the blade root. Therefore a modern rotor blade is twisted, which means that the chord line of the airfoils in the direction from tip to root is turning. The number of degrees of twist in the length direction is called the twist or the twist difference and is e.g. 10°-12° for classic blades. Thanks to the twist, the profiles are optimally adjusted to the flow, at least in the aerodynamic part of the blade. In the transition part, the inflow angle turns fast with decreasing distance to the axis. This fact together with the fact that optimum angle of attack for a profile decreases with increasing relative profile thickness, means that much twist is required to set the profiles at the correct angle. The large twist over a small radial distance (high twist rate) leads to structural problems and not to much extra energy and therefore the designer chooses not to further twist the transition part e.g. the part between 0% L and 20% L or that between 5% R and 20% R. An additional argument for this is that conventional computer codes to calculate the yield such as Blade Element Momentum codes or CFD codes estimate that adding twist in the transition part hardly has a yield benefit, e.g. less than 1%.
A final complication is that extreme loads to a wind turbine often occur at survival wind speed when the turbine is halted. Then the blade surface is the determining factor for the magnitude of the forces. Therefore the designer would like to choose the blade as small as possible, however due to moderate lift coefficients and relatively thick airfoils at the inner side of the rotor, he is forced to choose a large chord (in order to reach an acceptable energy yield), with consequently high loads at extreme wind speeds and thus a heavy and expensive structure.